Selective call communication systems that transmit two or more signaling formats on a single communication channel are well known in the communications art. Asynchronous formats, such as two-tone and five-tone sequential signaling, and short-term-synchronous formats, such as the Post Office Code Standardization Advisory Group (POCSAG) and Golay Sequential Code (GSC) selective call signaling, have been combined on common communication channels for many years.
Conventional POCSAG and GSC signaling formats require the transmission of a synchronization signal, or "prearnble," necessary for battery saving circuits in receivers operating in the system, and for synchronizing the receivers prior to the transmission of user information. Systems utilizing these formats typically send pages in batches, a batch being transmitted in response to either a predetermined number of pages having been queued for output, or a time limit for delaying queued pages having been reached, whichever condition is satisfied first. The batches thus can be sent at random times that are traffic dependent.
Long-term-synchronous signaling formats, such as the signaling format disclosed in PCT Publication No. WO 91/10304, by Michael J. DeLuca, published Jul. 11, 1991, are now emerging. Such formats transmit information in "frames" that occupy predetermined, cyclic positions in time. Selective call receivers operating with these formats are assigned to monitor specific ones of the frame positions for battery saving purposes. During time intervals corresponding to frame positions not assigned to a receiver, the receiver "sleeps" in a low-power mode to conserve battery power. Long-term-synchronous signaling formats can provide very long, e.g., four-minute, battery saving cycles when required for superior receiver battery life.
A problem can occur when one attempts to mix a short-term-synchronous signaling format and a long-term-synchronous signaling format on a common channel. The problem can occur, for example, if separate, interlocked encoders are used on the common channel, and the short-term-synchronous encoder is transmitting on the common channel at a time when an assigned frame position of the long-term-synchronous format arrives. Such undesirable interaction can cause missed or severely delayed pages in the system utilizing the long-term-synchronous format.
Thus, what is needed is a method and apparatus for sharing a long-term-synchronous signaling format and a short-term-synchronous signaling format on a common communication channel without producing undesirable interaction between the two formats. A method and apparatus is needed that will allow an existing communication system to make a graceful transition from a short-term-synchronous format to the more modern, long-term-synchronous format, while preferably allowing immediate exploitation of the superior battery saving characteristics of the long-term-synchronous format.